A. Technical Field
This invention relates to the sputter deposition of material. More particularly, it relates to the ion beam sputter deposition of thin films. Such thin films may be used for optical coatings or other applications.
B. Related Art
A comprehensive literature of the history of depositing multiple layer optical coatings (commonly called optical films) by ion beam sputtering can be found in David T. Wei, Harold R. Kaufman and Cheng-Chung Lee, `Ion Beam Sputtering,` Chapter 6 of `Thin Films for Optical Systems,` ed. by F. R. Flory, publ. by Marcel Dekker, New York, (1995) [Reference 1] patent relating to fabricating multi-layer optical coatings includes David T. Wei and Anthony W. Louderback, U.S. Pat. No. 4,142,958, `Method for Fabricating Multi-Layer Optical Films,` (1979) [Reference 2]
Up to this date, optical coatings made by ion beam sputtering have the lowest optical losses and are the most stable environmentally. In commercial production, they are also very uniform and reproducible lot-to-lot. Ion beam sputter-coated mirrors are crucial for ring-laser gyroscope (RLG) and laser interferometer gravitational-wave observatory (LIGO). Also, other ion beam sputter deposited coatings are in high demand for high precision laser instruments.
Optical coatings may have a rugate filter structure, which is a single layer of film having a continuously varying refractive index along a thickness direction with a number of minima and maxima in the index. A review of rugate filter structures is given in R. E. Johnson and R. L. Crane, "An overview of rugate filter technology," Int. Conf. Optical Interference Coatings, TuFl, pp. 118-121 (Tucson, Ariz., Apr. 12-15, 1988) [Reference 3]
The art of Sputter Deposition (sputtering for short) can be classified into two types: plasma sputtering and ion beam sputtering.
i. Plasma Sputtering
In this type, the main chamber of a vacuum system is maintained at a residual pressure around 10 millitorr by an inert, ionized sputter-gas (for example, argon) called a plasma. This plasma bombards a material target (for example, titanium) and sputters its surface off, atom-by-atom. The sputtered material is in turn deposited on another disc (called a substrate) as a thin film. In case of optical coating, the substrate should be optically transparent, and a tenuous plasma of reactive gas (for example, oxygen) is usually added to the sputter-gas to turn the film into a titanium dioxide film. Meanwhile, the titanium target's surface gets oxidized, too.
Plasma can be generated from the gas atoms by collision with electrons driven by either a direct current or a radio-frequency potential, which are traditionally called DC sputtering and RF sputtering, respectively. If the plasma bombardment is intensified by the use of strong magnetic field confinement, the set up is called magnetron sputtering.
Another technique using a plasma is plasma-assist coating. Plasma-assist coating is a hybrid using a plasma to bombard the thin film deposited by the traditional evaporation method in order to improve the film's micro-structure.
Plasma sputtering generally needs a certain concentration of gas molecules, typically 1 to 10 millitorr of residual pressure, to operate. At this concentration, intermolecular collisions are frequent, and undesirable contamination and charge damage to the film is more difficult to eliminate than at lower concentrations.
ii. Ion Beam Sputtering
In this class of sputtering, the deposition is the result of an ion beam bombarding a target material. The ion beam is a uniform flow of ions accelerated to a predetermined energy and is charge-neutralized. High optical quality ion beam sputtered coatings have gained recognition (Reference 2) since 1979, when a class of broad beam ion sources attributed to Kaufman (Reference 4) has been used. The Kaufman-type ion source is very uniform at its orifice and has a low level of contamination. Before this time, ion beam sources (such as duoplasmatrons) used very high energy ions (up to several Mev's) with a small beam diameter at its orifice. Those prior sources resulted in relatively low sputtering efficiencies and were mostly used to deposit metal films.
In a broad beam ion source (such as, but not limited to, any type discussed in Reference 1) a gas plasma is generated by electron collisions inside the source. The ions are then extracted by a voltage of tens to several thousand volts from the source into the main vacuum chamber through an orifice such as, but not limited to, cascaded grids each contains many holes, closely spaced and aligned. A neutralizer is located immediately outside the orifice to mix electrons with the ions. The neutralized ion beam can be operated in a vacuum environment of 0.1 millitorr without expansion until it hits the target, sputtering off portions of the target material. The neutralized ion beam can be generated by either a direct current (DC) or a radio frequency (RF) potential. In addition, a constituent gas from a chemical source is aimed at the deposit in situ. Together, the sputtered target material and the constituent gas form a transparent coating.
Advantages of the above described ion beam sputter deposition (IBSD) include its relatively high uniformity and reproducibility, its relatively strict neutrality, and its relative absence of gaseous contamination. The optical films it produces have both low losses in scattering and in absorption. The refractive indices of thin films are almost as high as corresponding bulk indices. Those of titanium and tantalum oxides are very high, making them well suited for high index layers in interference coatings. However, the sputter rate of such traditional IBSD is slow, making the cost per unit area too high for many applications.
Ion beam sputtering can be performed at a residual pressure range between 0.1 to 1 millitorr. In this pressure range, the residual gas concentration is so low that molecule-to-molecule collisions are practically absent, and the unwanted effects in plasma sputtering can be avoided.
iii. References
The following numbered references are herein incorporated by reference in their entirety:
1. David T. Wei, Harold R. Kaufman and Cheng-Chung Lee, `Ion Beam Sputtering,` Chapter 6 of `Thin Films for Optical Systems,` ed. by F. R. Flory, publ. by Marcel Dekker, New York, (1995)
2. David T. Wei and Anthony W. Louderback, U.S. Pat. No. 4,142,958, `Method for Fabricating Multi-Layer Optical Films,` (1979).
3. R. E. Johnson and R. L. Crane, "An overview of rugate filter technology," Int. Conf. Optical Interference Coatings, TuFl, pp. 118-121 (Tucson, Ariz., Apr. 12-15, 1988).
4. H. R. Kaufman, `Technology of Ion Beam Sources Used in Sputtering,` J. Vac. Sci. Technol., 15, 272-276 (1978).
H. Summary of the Invention
The ideas and design of the present invention seeks to remove a number of effects that slow down and degrade the optical coating in the prior art, and also maintain its superior quality. This invention still uses ion beam sputtering deposition (IBSD), but it also uses novel chemical reactions or partitions, or both, to assist the IBSD.
For example, in the prior art, the sputter yield was limited by gas molecules (e.g., oxygen) straying away from the deposition area, reaching the target material (e.g., titanium), and reacting with the surface of the target material to form a surface film. However, in the present invention, such reactions are substantially prevented by an in situ chemical spraying of an Assist Chemical on the target surface as the sputter deposition of the ion beam progresses. This results in an increased sputter yield of the target material.
The present invention includes the following features:
1. Assist Chemical: to have a second chemical reaction to overcome the side effect of the constituent gas reaction on the target surface. Its purpose is to assist the main ion beam to speed up the sputter yield.
2. Discriminate Baffle: to block the passage of certain unwanted species of particles, such as remained flow of assist chemical, from reaching the coating deposit. Alternatively, it can also serve as a barrier for any chemical intended for treating the deposit wandering to the other parts of vacuum.
3. Screening Chemical: to activate a third chemical reaction that surrenders any assist chemical which may reach the coating.
4. Compartmentalization: to divide the vacuum chamber into two physical separated compartments: one mainly for the ion beam sputter-deposition, another mainly for performing the constituent gas reaction, each with its own exit port for exhaust of ion and vapor (low.
5. Compartmentalized Assist Ion Beam: to aim additional, value-added assist ion beam(s) toward the deposit in the compartment for performing the constituent gas reaction. The added values of their usage are: (a) precision control of the coating reaction temperature and composition simultaneously. (b) refinement of the deposit material by generating a condensation differential of the molecules deposited (those desired and those undesired) eliminating the impurities therein, and (c) permitting adjustment of surface temperature and ion energy independent from the main ion beam by means of the compartmentalization,
6. Combination of and Multiple Use of the Resources Above. The spirit and scope of the invention includes the various possible combinations of the above features. Moreover, the said chemicals, ion beam sputtering sources, and the constituent gas, targets, together with their feed instruments, discriminate baffles and partitions, are not limited to one per each type, and can be rearranged within the vacuum chamber to suit a special purpose so as improve the coating rate, the throughput, or a greater variety of coatings.